Light boxes for X-rays are known in the prior art. They are essentially constituted by a translucent slab placed over lighting means. X-ray negatives to be examined are placed on the slab. The outlines and the structures which they represent are observed by differences in the attenuation of the light energy transmitted from the lighting means to the eye of the observer through the negative.
In stereotaxis examination for the purpose of determining the XYZ co-ordinates in three dimensions of a particular region of the body under examination (generally a lesion), two such negatives taken under two different angles of incidence are placed on the slab. Light boxes intended for this purpose also include moving spot index marks which make it possible to accurately locate points to be analyzed on the negatives. These index marks are generally driven by a mechanical device actuated by an operator. The operator actuates the mechanical device to bring the index marks into coincidence with the points to be analyzed on the film. A transcribing device records the mechanical displacements performed and transmits the corresponding position co-ordinates of the index mark to a computer. In conventional manner, the computer performs suitable processing to compute the three dimensional coordinates of a lesion on the basis of the positions of two index marks placed in like manner on each of the corresponding parts of the stereographic negative. The co-ordinates may be displayed on a video monitor or on any other appropriate means. They may also be used in a therapeutic treatment device or in a device for taking tissue samples for the purposes of cytological analysis.
In order to transmit the position of an index mark, the transcribing devices include a mechanical portion. The presence of this mechanical portion means that the transcribing devices have low accuracy. The exact position of the index mark is always transcribed with a degree of error because of bending or because of play. In addition, such transcribing devices are both expensive and heavy. Finally, because of their mechanical nature, they are liable to come out of adjustment and consequently they require frequent maintenance: cleaning, greasing, and recalibration. Such an apparatus is described in FR-A-2 248 535.
In addition, when examining stereographic negatives, it may be advantageous to have more than one pair of corresponding index marks available. Given the mechanical nature of the means for putting the index marks into place, it may be necessary to move a previously placed mark out of the way when placing a new mark because the mark-placing mechanisms, cannot overlap or crossover. That is why the maximum number of corresponding index mark pairs which can be used is small: the maximum known number being three.
The object of the invention is to remedy these drawbacks by providing a light box in which the slab and the lighting means beneath it are replaced by a graphics screen of one of the types now widely available commercially. In one example, the screen used is a matrix of light-emitting diodes (LEDs). It is also possible to use screens implementing other technologies: plasma screens; liquid crystal screens; or even cathode ray tubes.
Nevertheless, the graphics screen is used in a way which is different from the way in which such screens are normally used. A graphics screen normally comprises a plane which is not very bright: brightness occurs only at those locations which are to represent drawing lines. The invention proceeds differently. The screen is caused to emit light all over. It becomes dark only at points corresponding to the positions at which index marks are to be located. The screen must therefore have sufficient brightness to be about as bright as a current light box. In addition, the outside surface of the screen which is being used as a slab should preferably be flat. This means it can receive X-ray negatives without warping them.
A graphics screen is essentially characterized by its suitability for enabling the positions of its inverse brightness pixels (i.e. dark points) to be determined in an associated frame of reference. This may be achieved by any appropriate means. For example, position may be determined as a function of time within a frame of a video signal being displayed on the screen, with the time position of the pulse in this signal which corresponds to the dark portion being measured. If an image memory is used, then the addresses (the co-ordinates) of the dark points on the screen are directly related to the addresses of those memory cells in the image memory which contain information about the dark points (e.g. which contain a zero bit) unlike the remainder of the image which is bright (e.g. corresponding to one bits).